1. Field of the Invention
The present invention relates to fluorescence for an image display apparatus which is used for a color television receiver, a terminal display of a computer and the like.
2. Description of the Related Art
As a panel-type color display apparatus for displaying a color image, there is a color display apparatus which utilizes cathode luminescence as disclosed in Japanese Patent Application Kokai (Laid-Open) No. JP-A-57-135590. This display apparatus will be explained below. FIG. 4 shows the basic configuration of this display apparatus. Configuration elements of the apparatus are, in order from the backside to the front, that is, in the order from the left side to the right side in FIG. 4, a back electrode 51, a linear cathode 52 as a beam source, vertical focusing electrodes 53a and 53b, a vertical deflection electrode 54, a beam modulation electrode 55, a horizontal focusing electrode 56, a horizontal deflection electrode 57, a beam acceleration electrode 58 and a screen 59, which are all accommodated in a vacuum flame glass bulb (not shown). The linear cathode 52 as a beam source is stretched or elongated in a horizontal direction so as to generate an electron beam which linearly distributes in the horizontal direction, and a plurality of linear cathodes 52 are provided in a vertical direction with a suitable distance therebetween (only four linear cathodes 52a.sub.1 to 52d.sub.1 are shown in the drawing). Assume that there are fifteen such linear cathodes provided in this case. These linear cathodes are structured by coating an oxide cathode material on the surface of a tungsten wire of 10 to 20 .mu.m.phi., for example. These linear cathodes are so controlled that an electron beam is emitted from each of them for a predetermined time sequentially starting from the linear cathode 52a.sub.1, as described later. The back electrode 51 restricts generation of an electron beam from the linear cathodes 52 other than the linear cathode 52 which is being controlled to emit an electron beam for a predetermined time, and the back electrode 51 also operates to transmit the generated electron beam only in the forward direction. The back electrode 51 may be formed by coating a conductive material on the inner surface of the rear wall of the glass bulb. Instead of the electron beam source which is constituted by the linear cathode 52 and the back electrode 51, a plane electron source may also be used. The vertical focusing electrode 53a is a panel-shaped electrode which has a long slit 60 in the horizontal direction, facing each of the linear cathodes 52a.sub.1 to 52d.sub.1. The vertical focusing electrode 53a takes out the electron beam emitted from the linear cathode 52 through the slit, and focuses the beam in the vertical direction. The slit 60 may be constructed by crosspieces arranged at suitable intervals, or by a string of many piercing holes arranged at small intervals in the horizontal direction. The vertical focusing electrode 53b also has a similar structure.
A plurality of vertical deflection electrodes 54 are disposed in the horizontal direction at intermediate positions of the slits 60. Each of the vertical deflection electrodes 54 has conductors 63a and 63b which are disposed on the upper and lower surfaces of an insulation substrate 62 respectively. A vertical deflection voltage is applied between the facing conductors 63a and 63b, and the electron beam is deflected in a vertical direction. In this case, an electron beam from one linear cathode is deflected in a vertical direction at positions of sixteen lines by a pair of conductors. Fifteen pairs of conductors corresponding to the fifteen linear cathodes 52 are structured by sixteen vertical deflection electrodes 64. As a result, electron beams are deflected so that 240 horizontal lines are drawn on the screen 59.
Each of the beam modulation electrodes 55 is constituted by a strip-shaped electrode having a slit in the vertical direction, and a plurality of the beam modulation electrodes 55 are arranged in the horizontal direction with a predetermined distance therebetween. In this case, 320 beam modulation electrodes 55a to 55n are provided (only ten beam modulation electrodes are shown in the drawing.) Each of the beam modulation electrodes 55 separates the electron beam in each one picture element and takes it out in the horizontal direction, and modulates the quantity of electron beams passed by an image signal for displaying 320 beam modulation electrodes 55, it is possible to display 320 picture elements per one horizontal line. Each picture element is displayed by a phosphor of three colors including R, G and B in order to make a color image display. Each image signal of R, G and B is sequentially added to each beam modulation electrode. 320 sets of image signals for one line are simultaneously applied to the 320 beam modulation electrodes 55, and the image for one line is displayed simultaneously.
The horizontal focusing electrode 56 is a panel-shaped electrode 67 which has a plurality (320 pieces) of slits 66 elongated in the vertical direction, facing the slits 64 of the beam modulation electrode 55. The horizontal focusing electrode 56 focuses, in the horizontal direction, each of the electron beams for each picture element separated in the horizontal direction, and forms a fine electron beam.
The horizontal deflection electrode 57 is constituted by a plurality of conductive panels 68 which are disposed in the vertical direction at intermediate positions of the respective slits 66. A horizontal deflection voltage is applied between the respective conductive panels 68 to deflect an electron beam of each picture element in the horizontal direction and to sequentially irradiate each of the phosphors R, G and B to produce light emission on the screen 59. The deflection width in this case is the width of one picture element for each electron beam.
The acceleration electrode 58 is constituted by a plurality of conductive panels 69 which are provided in the horizontal direction at positions similar to the positions of the vertical deflection electrode 54, and the acceleration electrodes 58 accelerate electron beams so that the electron beams impinge on the screen with sufficient energy.
The screen 59 is constituted by a glass panel 71 whose rear surface is coated with a phosphor 70 that emits light by the irradiation of an electron beam, and also by a metal back layer (not shown). A pair of phosphors 70 which includes three colors of R, G and B are provided for one slit 64 of the beam modulation electrode 55, that is, for each one electron beam separated in the horizontal direction, and the phosphors are coated in a stripe shape in the vertical direction. In FIG. 4, broken lines entered in the screen 59 show sections in the vertical direction which are displayed corresponding to each of the plurality of linear cathodes 52, and two-dotted chain lines show sections in the horizontal direction which are displayed in correspondence with each of the plurality of beam modulation electrodes 55. Each one section separated by these lines include the phosphor 70 (G, R, G) for one picture element in the horizontal direction and a width of 16 lines in the vertical direction, as shown in an enlarged drawing in FIG. 5. The size of one section is, for example, 1 mm in the horizontal direction and 16 mm in the vertical direction.
It should be noted that, in FIG. 4, the length in the horizontal direction is shown to be much larger than the length in the vertical direction, to facilitate understanding.
Although only one pair of phosphors 70 for R, G and B are shown for only one picture element of one beam modulation electrode 55, that is, for one electron beam, in this case, two or more pairs of phosphors for two or more picture elements may be provided, in which case image signals of R, G and B for the two or more picture elements are sequentially applied to the beam modulation electrode and horizontal deflection is also performed in synchronism with this operation.
The display apparatus according to the prior art, however, has the following problems. There occurs a positional deviation between the pitch in the horizontal direction of an electron beam irradiated on the screen and the phosphor stripe pitch, which is attributable to a positional deviation between the phosphor stripe pattern on the screen 59 and the electrode groups which comprises the beam modulation electrode 55, horizontal focusing electrode 56, horizontal deflection electrode 57 and other electrodes.
One of the causes for the above problems is positional deviation between the screen and the electrode group in the process of fabrication of a display apparatus. For example, the screen 59 is formed on the glass panel, and the glass panel usually contracts whenever it undergoes a heat process and has a possibility of contraction by tens of .mu.m in the case of a glass panel which has a length of 30 to 40 cm. The value of contraction is not constant. Accordingly, there occurs a change in the pitch of the phosphor stripe pattern.
A second cause is a thermal expansion difference between the screen 59 and the electrode group at the time of displaying an image, 42--6 alloy (42% Ni, 6% Cr, balance Fe) and the like, of which coefficient of thermal expansion is close to that of glass, is used as the material for the electrode group, but it is difficult to maintain both the electrode group and the screen at the same temperature in the image display state. Therefore, there occurs a deviation between the pitch in the horizontal direction of the electron beam irradiated on the screen and the phosphor stripe pitch. There is also a possibility that this deviation changes with time due to temperature changes.
A warp of the electrode group or the screen is also another cause. In respect of the individual structure of the electrode group and the screen, the slit pitch precision of the electrode group and the phosphor stripe pitch precision of the screen will also become a problem.
For the above reasons, the pitch in the horizontal direction of the electron beam irradiated onto the screen does not match the phosphor stripe pitch. When the electron beam and the phosphor stripe in the horizontal direction are positioned at the center in the horizontal direction of the display apparatus, pitch errors are accumulated at both ends and there occurs a color deviation at the center.